1. Field of the Invention
The present invention relates to a deterioration detecting device for an oxygen concentration sensor disposed downstream of a catalyst, which is provided in an exhaust passage of an internal combustion engine to purify exhaust gases.
2. Description of the Related Art
A catalyst for purifying exhaust gases is provided in an exhaust passage of an internal combustion engine to perform purification of the exhaust gases. Further, an oxygen concentration sensor (which sensor will be hereinafter referred to as “downstream oxygen concentration sensor”) is disposed downstream of the catalyst for purifying exhaust gases. Conventionally known is a system which performs deterioration determination of the catalyst according to a detection signal from the downstream oxygen concentration sensor, and also performs air-fuel ratio control for utilizing maximum purifying ability of the catalyst.
If the downstream oxygen concentration sensor is deteriorated, the deterioration determination of the catalyst and the air-fuel ratio control cannot be properly performed. Therefore, various methods for determining deterioration of the downstream oxygen concentration sensor have been proposed. One example of the proposed methods is described in Japanese Patent Laid-open No. Hei 10-169494. According to this method, when a target air-fuel ratio is changed from a rich air-fuel ratio to a lean air-fuel ratio, the time period (output change time period) from the time the output from the downstream oxygen concentration sensor starts to change from a rich region to a lean region to the time the amount of change in the output from the sensor becomes a predetermined amount, is detected. When this output change time period is greater than or equal to a predetermined time period, it is determined that the downstream oxygen concentration sensor is deteriorated.
In this determination method, the predetermined time period used as a threshold for the deterioration determination is constant. Accordingly, when the catalyst performance is deteriorated by a sulfur component contained in the fuel, or when an engine operating condition changes, there is a high possibility of improper determination. That is, when the amount of the sulfur component accumulated in the catalyst becomes larger, or when the flow rate of exhaust gases from the engine is small, an output change speed of the downstream oxygen concentration sensor after changing the air-fuel ratio tends to become slow (the output change time period tends to become long), even if the downstream oxygen concentration sensor is normal. Thus, the downstream oxygen concentration sensor may be improperly determined to be deteriorated, when it is actually normal.